Estrogen receptors (ER) belong to the family of nuclear hormone receptors. Nuclear hormone receptors define a superfamily of ligand-activated transcription factors (Evans, 1988, Science 240:889). Members of this family are typically characterized by the presence of conserved modular domains: a zinc finger DNA-binding domain (DBD) triggers the interaction of the receptor with specific response elements at the DNA site, a ligand-binding domain (LBD) adjacent to the DBD, and two transcriptional activation domains AF-1 and AF-2 ligand independent and ligand dependent, respectively (Nilsson, 2002, SERMs: Research and clinical applications, Eds: Humana Press Inc, 3). Upon ligand binding to the receptor, a conformational change occurs within the LBD bringing the AF-2 domain in closer proximity and allowing for the recruitment of co-activators. Co-activators create a physical interaction between the nuclear-hormone receptor and components of the transcriptional machinery, establishing transcriptional modulation of target genes.
Two estrogen receptor subtypes have been identified: ER alpha (ERα, NR3A1) (Green, 1986, Nature 320:134; Greene, 1986, Science 231:1150) and ER beta (ERβ, NR3A2) (Kuiper, 1996, PNAS 93:5925). Both receptors bind to the endogenous natural ligand 17β estradiol with comparable high affinity and modulate the transcriptional activity of target genes through classical estrogen response elements (reviewed in Nilsson, 2005, Bas Clin Pharm Tox, 96:15). More recently, it has been demonstrated that estrogen receptors can mediate non classical actions (reviewed in Osborne, 2005, J Clin Oncol 8:1616): (1) non-classical transcriptional regulation in which ERs function as co-activators on alternate regulatory DNA sequences, (2) non genomic or membrane-initiated steroid signaling in which ERs evoke rapid cytoplasmic signaling, and (3) crosstalk with Receptor Tyrosine Kinases (RTKs). Interestingly, their ligand binding domains (LBD) only share 56% amino acid identity which suggest that they might accommodate different ligands and thus mediate different or even opposite effects (Kuiper, 1997, FEBS Lett, 410:87). Moreover, the distribution pattern of the two receptors is quite different (reviewed in Mathews, 2003, Mol Intery 3:281). Both ERs are widely distributed both peripherally and in the brain, displaying distinct and sometimes overlapping patterns in a variety of tissues. ERα is expressed primarily in the uterus, liver, kidney and heart. On the other hand ERβ is present mainly in the ovary, prostate, lung, gastrointestinal tract, bladder, hematopoietic and central nervous system (CNS). ERβ specific localization in the CNS includes the hippocampus and thalamus (Osterlund, 2001, Prog Neurobiol 64:251; Ostlund, 2003, Ann NY Acad Sci 1007:54). ERα and ERβ are co-expressed in the mammary gland, epididymis, thyroid, adrenal, bone and the dorsal root ganglia of the spinal cord and the cerebral cortex of the brain.
The characterization of mice lacking ERα or ERβ has provided insight into the physiology of estrogen receptors (reviewed in Hewitt, 2000, Breast Cancer Res 2:345; Couse, 1999, Endoc Rev 20:358). Both ERα male and female null mice are infertile because of dysfunction in spermatogenesis and ovulation, respectively. In addition, null females display a lack of sexual behavior, increased aggression and infanticide. Null male exhibit normal mounting behavior but a complete lack of intromission and ejaculation. They also show reduced aggression. In contrast, ERβ null female mice are subfertile with reduced littermates. Male counterparts show no apparent defects in their reproductive tract. The neuroendocrine system is significantly altered in ERα null mice in contrast to ERβ null mice which do not show any impairment. Moreover, the knock-out of ERα in mice leads to absence of breast tissue development, lower bone density and impaired glucose tolerance. Knock out studies of ERβ led to controversial results with some studies being unable to see an effect on bone density (Lindberg, 2002, J Bone Min Res 17:555), whereas other reports suggested an increase in trabecular bone volume in females only due to decreased bone resorption (reviewed in Windahl, 2002, Trends Endoc Metab, 13:195). Interestingly, morphological alterations in the brains of mice lacking ERβ are evident (Wang, 2001, PNAS 98:2792) including an association with impaired neuronal survival (Wang, 2003, PNAS 100:703). This has led to speculation that ERβ could have an important role in protecting from neurodegenerative disorders such as Alzheimer and Parkinson diseases, and potentially those resulting from trauma and cardiovascular insults. This hypothesis is further supported by experimental studies indicating a neurotrophic and neuroprotective role for estrogens (reviewed in Wise, 2002, Trends Endocrinol Metab 13:229; Behl, 2003, J Steroid Biochem Mol Biol 83:195).
More recently, the use of a relatively selective ERβ agonist has unraveled a prominent role in inflammation for this subtype (Harris, 2003, Endoc 144:4241). Beneficial effects were seen in animal models of inflammatory bowel disease and adjuvant-induced arthritis. Indeed, ERβ is expressed both in the intestine and in immune cells. Moreover, ERβ null studies have suggested a role in thymus function (Erlandsson, 2001, Immunol 103:17) as well as in pulmonary inflammation (Patrone, 2003, Mol Cell Biol 25:8542). Interestingly, though, no effects associated with classical estrogen function were evident through the use of this ERβ agonist (Harris, 2003, Endoc 144:4241). In particular, that ligand was inactive in mammotrophy, bone density and ovulation in in vivo assays. This data is contrary, to a certain extent, to a variety of studies including human polymorphisms, knock-out animals, and tissue distribution that argue for a role of ERβ in bone and ovulation homeostasis. Other proposed therapeutic roles for selective ERβ agonists include prostate and breast cancer, autoimmune diseases, colon cancer, malignancies of the immune system, neurodegeneration, cardiovascular function, and bone function (reviewed in Koehler, 2005, Endocr Reviews, DOI 10.1210). Several ERβ agonists are discussed in the International Publications WO 2005/108337 and WO 2007/0565500.